A shockingly intense heat wave for the location, and for so early in the year, will produce some of the highest readings ever observed across much of the Northwest U.S. and adjacent southwest Canada. Temperatures will soar to dangerous levels – well above 100 degrees Fahrenheit in many places – from Friday, June 25, into the following week, in a region where hundreds of thousands of residents lack central air conditioning or any AC at all.

For several days, multiple computer forecast models have been spitting out astonishing numbers for Portland, Oregon; Seattle and Spokane, Washington; Vancouver, British Columbia; and other towns and cities. Even with some potential model overestimation, confidence is growing that a truly historic heat wave is on tap.

One sign of this is official forecasts from the National Weather Service: They’ve grown bolder through the week as model agreement has solidified and the event has drawn closer. As of midday Thursday, June 24, the National Weather Service forecast was calling for Spokane to hit 110°F on both Monday and Tuesday, June 28 and 29. These would break the city’s all-time high of 108°F from July 26, 1928, and August 4, 1961.

In Portland, Sunday is predicted by the National Weather Service to be the hottest day in city history. The forecast high of 109°F would topple the all-time record of 107°F set on July 30, 1965, as well as August 8 and 10, 1981. (See more on local records below.)

It’s extremely unusual for the National Weather Service to predict three or four days in advance that all-time records could be not only approached but exceeded. Such is the projected intensity of this heat wave and the resounding agreement among the world’s top forecast models.

In some cases, the highest single-day low temperatures ever recorded may be challenged as well. Warm nights only add to the danger of multi-day heat waves.

Map of Pacific Northwest region
Temperatures will be 20 to 28 degrees Celsius (36 to 50 degrees Fahrenheit) above average at 5 p.m. PDT Sunday, June 28, 2021, over much of the Pacific Northwest coastal region, according to the 12Z Thursday, June 24, run of the GFS model. (Image credit: tropicaltidbits.com)

“This will be an *exceptionally dangerous* heatwave from a public health perspective, especially since this is a part of the country where structures are not designed to shed heat and where air conditioning is rare,” tweeted climate scientist Daniel Swain (@Weather_West). “Infrastructure/ power disruption is also possible.”

What’s even more astounding and concerning is the timing of this event, weeks earlier than anything comparable in the past. Heat waves tend to be more dangerous when they occur early in the summer, before people have had time to fully acclimate to high temperatures.

Seattle’s official definition of a heat wave is at least three consecutive days topping 90°F. Such a streak has never before occurred in June, when clouds and cool air more often enshroud the city, delaying summer warmth and producing what’s known locally as “June gloom”. However, the city’s first June heat wave on record is almost certain to unfold from Saturday to Monday, June 26-28, with predicted highs of 95°F, 99°F, and 99°F.

Extreme as they sound, these forecasts might need to be adjusted even higher. As of early Thursday, June 24, a consensus product of models used by the National Weather Service (the National Blend of Models) was giving Seattle a high of 101°F on Sunday and 104°F on Monday. In Portland, the blend was calling for a Sunday high of 111°F.

The heat will be no less impressive north of the U.S.-Canada border. Temperatures in Vancouver, British Columbia, could reach 31°C (88°F) in town by Tuesday, June 29, according to Environment Canada. Readings at or above 37°C (99°F) are possible on the warmer east side of the metro area. Vancouver’s all-time high is 34.4°C (93.9°F). According to international weather records researcher Maximiliano Herrera, 36.0°C (96.8°F) was recorded at the West Vancouver station.

Farther east in British Columbia, the interior city of Kamloops is predicted to hit 40-41°C (104-106°F) on each day from Sunday through at least Wednesday. The city’s all-time high is 41.7°C (107.1°F), and the all-time record for Canada is 44.4°C (111.9°F), set at several locations.

The regional heat wave should abate in cities near the coast starting on Tuesday, but temperatures will remain mostly above average throughout the week. Meanwhile, the core of the heat will move eastward into Idaho and western Montana, possibly leading to all-time highs in that region as well.

What’s driving the heat wave?

As with most large-scale heat waves, the culprit in this case is an exceptionally strong upper-level high predicted to develop across the region. The high is partially a result of a pocket of rich moisture in the Northwest Pacific, with reverberations translating eastward along the jet stream and forcing the unusually strong upper high to develop, according to meteorologist Philippe Papin.

Another factor is the unusually parched landscape of eastern Washington, Oregon, and British Columbia. The dryness will allow energy from intense sunlight to bake the ground rather than evaporating ground moisture. Since March 1, Spokane has received just 1.10 inches of rain – the least moisture for any spring and summer to date since 1924. Every other such period in the last century has seen at least twice as much rain in Spokane. Wheat farms across eastern Washington, most of which lack irrigation, are suffering in the drought.

By one common measure, the heat dome at the center of this upper-level high will be as strong as any ever observed in the Pacific Northwest, if not stronger (see embedded tweet below). As warm air expands at lower levels, it pushes up the height of the 500-millibar surface, roughly at the midpoint of the atmosphere vertically. The 500-mb height is predicted by the European and GFS ensemble model averages to exceed 594 decameters from Washington state northward into Canada. The record-high 500-millibar height at Quillayute, Washington, as measured in twice-daily weather-balloon launches (soundings) since 1948, is 597 dm; no event before mid-July has seen anything higher than 593 dm.

The urban and rural impacts of Northwest heat

Cities such as Portland, Seattle, and Vancouver are especially vulnerable to heat impacts. Most of the time, marine air dominates these regions, keeping conditions mild in summer and cool in winter. Just east of the mountains, though, the expansive semi-arid grasslands of eastern Oregon, Washington, and British Columbia are typically much warmer in summer and colder in winter.

On those rare occasions when the marine air is pinched off and an upper-level high sets in, even cities near the coast can heat up almost as much as the interior.

Unlike people in the South, where air conditioning is ubiquitous among all but the poorest residents, many folks in the Pacific Northwest lack AC at home. Low-income residents lacking the means to pay for air conditioning – disproportionately people of color – may also be more likely to live in the hotter parts of Pacific Northwest cities, where pavement and concrete are more prevalent and trees and green space are harder to come by.

Many of these areas overlap with historically Black neighborhoods that were “redlined” in the early and mid-20th century, as analyzed by a 2020 study in the peer-reviewed journal Climate. The redlining, a discriminatory practice of denying housing loans to people living in predominantly Black neighborhoods, was accompanied and followed by urban renewal and highway expansion that only increased the heat-island potential of many such areas.

Among the 108 urban areas analyzed, Portland had the greatest contrast in landscape temperatures (7.1°C or 12.8°F) between the highest- and lowest-rated of the four historical redlining categories.

Study coauthor Vivek Shandas of Portland State University, who’s now involved with several follow-up studies, said he and his colleagues in the Northwest are already pondering what the upcoming heat wave might mean for policy. “We’re getting potentially the hottest day in recorded history in the city of Portland,” Shandas said. “Are we heading into a longer stretch of these trends being more consistent?”

According to Shandas, the number of buildings with central AC is growing at a more rapid pace in Portland than Seattle, especially in new developments. Statistics are lacking on the use of portable air conditioners, but many local appliance stores are “flush out of ACs and have waiting lists,” he said. “I know there’s a run on them this weekend.”

In rural parts of the Pacific Northwest, the wildfire threat will climb as the upcoming heat wave exacerbates the impact of severe to extreme drought. There’s also some potential next week for “dry” thunderstorms, the type that produce little rain but can spit out lightning that sets off wildfires. “Fire weather concerns will certainly increase in the west when the ridge breaks down,” warned the National Weather Service Storm Prediction Center in its extended Day 3-8 fire weather outlook issued on Wednesday, June 23.

The heat will be especially protracted in eastern Washington, where nighttime lows may not dip below 70°F for days on end. Even where urban heat islands aren’t an issue, such an extended stretch of warm nights and hot days will make it increasingly harder for people to stay cool.

“This event will likely be one of the most extreme and prolonged heat waves in the recorded history of the Inland Northwest,” warned the Spokane office of the National Weather Service.

Smoke over Cascades, 9/11/20
Smoke from fires over Mount Baker–Snoqualmie National Forest, Washington, on September 11, 2020. (Image credit: Bob Wick/BLM via Flickr)

Will heat waves get disproportionately worse?

Longer and more intense heat waves are one of the most confident projections from models of human-caused global change, as noted in the Intergovernmental Panel on Climate Change’s 2013 Fifth Assessment Report. Already, U.S. heat waves since the 1960s have become more frequent and prolonged.

As climate change progresses, it was once assumed that the worst heat waves will intensify more or less in lockstep with overall temperatures. In other words, a 3°F increase in overall climate warming would lead to heat waves that are roughly 3°F hotter.

There’s been more debate on this point over the last decade, though, with some scientists arguing that hot-side variability is likely to increase as temperatures rise.

One graduate student working on heat-variability research is also at the center of this weekend’s Northwest heat wave. Lucas Vargas Zeppetello of the University of Washington is lead author of a 2020 paper in the Journal of Climate that uses a new modeling approach to find that peak summer temperatures are likely to increase even more quickly than average temperature, in large part because of the heating boost provided by dry ground in normally moister areas. Summertime temperature variability increases by as much as 10% in some parts of North America for a 4°C (7°F) warming, according to the analysis.

Vargas Zeppetello said that landscapes dried out by hot weather can swing disproportionately to extremely high temperatures. “It’s been happening all over the world, especially in Europe a couple of summers ago,” he said. An onslaught of early-season record-smashing heat in June 2019 produced all-time highs in more than 30 European towns and cities.

As for the upcoming heat wave in his home city of Seattle, “the models are all singing the same song, and the physics behind it is pretty straightforward,” he said. “In some ways this is like a textbook case of what I research. That doesn’t make it any less jarring when it actually happens.”

He added, “To see it play out like this is honestly pretty frightening.”

Records at risk

Seattle, June record:

100°F downtown (6/9/1955) (records kept here from 1894 to 1972)

96°F at Sea-Tac (6/25/2017)

93°F at the NWS office at Sand Point, near Lake Washington (6/25/2017)

Seattle, all-time record:

103°F at Sea-Tac Airport (7/29/2009) (records begin in 1948)

105°F at Sand Point (7/29/2009)

Note: these are the only officially recorded temperatures in Seattle weather history that have exceeded 100°F.

Portland, June record:

102°F (6/26/2006)

Portland, all-time record:

107°F at Portland International Airport (7/30/1965, 8/8/1981, 8/10/1981)

Note: According to weather historian Christopher Burt, the highest reading in earlier Portland weather data extending back to 1875 was a July reading of 102°F.

Spokane, all-time record:

108°F (7/26/1928)

Spokane, June record:

105°F (6/28/2015)

Thanks to Jeff Masters, Christopher Burt, and Maximiliano Herrera for contributions to this post.

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Bob Henson is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance...